Transcript 08B Lecture 2006

Major life history traits
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1
2
3
4
Age of Maturity
Fecundity
Parity (# times reproduce)
Aging and lifespan
1) Age of Maturity
When should an organism begin to breed?
*** Summarize the major result.
What explains the pattern?
What determines age of maturity?
• Affects generation time and rate of entry
of genes into gene pool
• Benefit to not delay: immediate fecundity
• Benefit to delay: (if have relatively long
lifespan)
 may have age-related gains in
fecundity from growth or experience
• BUT cost to delay:
•
May have risk of mortality with time
•
May have reduced fecundity at later
ages
***Is there a fecundity advantage by
delaying age at first reproduction? When
should an individual with a lifespan of 3
begin to reproduce? A lifespan of 7?
Reproductive value (Vx): takes into account
prob. survival and future reproduction.
It rises then falls during lifetime.
Natural selection is strongest at highest Vx.
.
2) Fecundity: How many offspring per
reproductive bout?
• Fecundity vs. parental
investment/offspring
•
seed size vs. seed number
•
egg size vs. egg number
• Variation in seed and egg size among
species
Growth vs. Fecundity
• If indeterminate growth,
• Fecundity is related to body size;
• Increased fecundity in one year reduces
growth, and thus fecundity, in future.
• Short-lived emphasize fecundity over growth
• High extrinsic adult mortality rates favor increased
reproductive effort, or investment in offspring, at
expense of adult survival and future reproduction.
• Long-lived emphasize growth over fecundity
3) Parity
• How many times to reproduce per
lifetime?
• Semelparous
• (monocarpic) once
• Iteroparous
• (polycarpic) repeated
If semelparous, at what year to undergo
‘big-bang’ reproduction?
• Annual
• Biennial
• Long-lived
• Semelparity: When payoff for reproduction is
highly variable but favorable conditions are
predictable from environmental cues?
• When pollinators attracted to massive display?
• When seed predators become satiated?
Semelparity: When preparation for
reproduction is extremely costly?
Iteroparity: When low current reproduction
results in maintaining high future
reproduction.
Annual vs. Biennial vs. Perennial…
• How much fecundity is required so number of
offspring of annual > perennial?
• FA > FP + Sadult/Sjuvenile
• How high fecundity is required for delayed
semelparity > no delay?
• Fdelayed > (Sjuvenile x Fannual) # yr delay
• What is proportion of annuals, biennials,
perennials? Which is most rare? Why?
4) Aging and Lifespan
• Senescence is a decline in
physiological function with age.
• Causes decline in fecundity and
survival
Hypotheses
• Antagonistic pleiotrophy
good early tied to bad late
• Accumulation of harmful mutations
Why does aging vary?
• Not all organisms senesce at same
rate, suggesting that aging may be
subject to natural selection and
evolutionary modification.
• Strength of selection diminishes on
traits expressed at progressively
later ages.
Strength of selection varies with
mortality rate. If high mortality, few
reach old age
little selection for mechanisms to
prolong life.
Individuals in populations with high
extrinsic mortality rates age faster.
*** Sample exam question
Life history traits often represent ‘tradeoffs’.
Graph the expected relationship for each pair
of traits. Label axes. Then explain the
nature of each tradeoff.
A. Seed size vs. seed number
1. Graph 2. Explanation
B. Number of offspring per breeding attempt
vs. adult survival between successive
breedings
1. Graph 2. Explanation
Objectives
• Define life history
• How to study life history?
• Correlated life history traits in contrasting
environments
• Resource allocation and tradeoffs
• Age of maturity
• Fecundity
• Parity (no. times reproduce/lifetime)
• Aging and lifespan
Vocabulary
trade-offs
parity
indete rminate growt h
monocarpy
bet hedgin g
r selected*
reproductive value
pollen limitation
perennial
resource allocation
fecundit y
programmed death
iteroparity
senescence
K selected*
parental investment
annual
maturity
aging
semelparity
polycarpy
clutch size
resource limitation
biennia l